GPT-5.6 Sol vs GPT-6 Luna
Pricing, context window and real answers (September 2026)
GPT-5.6 Sol (OpenAI) and GPT-6 Luna (OpenAI), compared as you can actually call them on FastMetal. Both are served from the same OpenAI-compatible endpoint and API key; switching is a change to the model string.
Specs and pricing
 GPT-5.6 Sol | GPT-6 Luna | |
|---|---|---|
| Provider | OpenAI | OpenAI |
| Input (per 1M tokens) | ¥357.4 | ¥17.87 |
| Output (per 1M tokens) | ¥1,787 | ¥89.35 |
| Typical cost (1,000 in + 500 out tokens × 1,000 calls) | ¥1,251 | ¥63 |
| Context window | 1,050,000 tokens | 1,050,000 tokens |
| Release date | 7/9/2026 | 9/22/2026 |
| Input modalities | file, image, text | file, image, text |
| Throughput (p50) | — | — |
| Arena · overall | #18 · ELO 1,483 | Unranked |
| Arena · Japanese | #5 · ELO 1,503 | Unranked |
| Arena · coding | #17 · ELO 1,528 | Unranked |
Prices are FastMetal's yen rates before tax. Arena ranks are as of the latest leaderboard scrape.
Which should you pick?
- On price, GPT-6 Luna: ¥89.35 per 1M output tokens, 95% less than GPT-5.6 Sol.
- If in doubt, try both on the same key. Switching is a change to the model string, and each is billed at its own rate.
Real answers to the same prompts
Exactly what the FastMetal gateway returned, side by side. Not benchmark scores: actual output.
Count the number of 'r's in 'strawberry'
Count the number of 'r's in 'strawberry'. Explain your reasoning step by step.
GPT-5.6 Sol
“strawberry” can be split into letters:
s, t, **r**, a, w, b, e, **r**, **r**, y
The letter **r** appears **3 times**.
3.0s78 output tokens
GPT-6 Luna
In **“strawberry”**, the letter **r** appears at positions 3, 8, and 9. So there are **3** “r”s.
2.7s94 output tokens
Debug This Error
I'm getting the following error in my Node.js application: TypeError: Cannot read properties of undefined (reading 'map') at UserList (/app/components/UserList.js:12:25) at renderWithHooks (/app/node_modules/rea…
GPT-5.6 Sol
The error occurs because `users` is `undefined` when `UserList` renders, so JavaScript cannot call `.map()` on it. This often happens when data is loaded asynchronously or the parent does not pass the prop.
### Fix options
**1. Give `users` a default value:**
```jsx
function UserList({ users = [] }) {
return (
<ul>
{users.map(user => (
<li key={user.id}>{user.name}</li>
))}
</ul>
);
}
```
Note that this handles `undefined`, but not an explicitly passed `null`.
**2. Handle missing or invalid data explicitly:**
```jsx
function UserList({ users }) {
if (!Array.isArray(users)) {
return <p>No users available.</p>;
}
return (
<ul>
{users.m…
7.1s395 output tokens
GPT-6 Luna
`users` is `undefined` when `UserList` renders, so calling `users.map(...)` throws. This often happens because the parent didn’t pass the prop or because data hasn’t loaded yet.
If an empty list is acceptable until the data arrives, provide a default:
```jsx
function UserList({ users = [] }) {
return (
<ul>
{users.map(user => (
<li key={user.id}>{user.name}</li>
))}
</ul>
);
}
```
Also check that the parent passes `users`:
```jsx
<UserList users={users} />
```
If `users` is loaded asynchronously, initialize it as an array:
```jsx
const [users, setUsers] = useState([]);
```
If `null` is also possible, normalize it before mapping, for example `(users ?? […
3.8s289 output tokens
Code Review
Please review the following Python function and suggest improvements for readability, performance, and best practices: def get_data(url, retries=3): import requests import time for i in range(retries):…
GPT-5.6 Sol
### Issues and suggested improvements
- Move imports to module scope.
- Avoid bare `except:`; it catches programming errors and interrupts such as `KeyboardInterrupt`.
- Use `response.raise_for_status()` instead of manually checking only for `200`; other 2xx responses may also be valid.
- Retry only transient failures, such as timeouts, connection errors, `429`, and selected `5xx` responses. Retrying most `4xx` errors is not useful.
- Use exponential backoff rather than a fixed delay.
- Avoid sleeping after the final failed attempt.
- Replace `item["active"] == True` with `item.get("active") is True`.
- Use a list comprehension for the transformation.
- Add type hints and validate arguments…
33.1s1646 output tokens
GPT-6 Luna
Key improvements:
- Move imports to module scope.
- Catch `requests.RequestException` instead of using a bare `except`; the current handler can hide programming errors as well as network failures.
- Call `raise_for_status()` so unsuccessful HTTP responses are handled explicitly.
- Use a list comprehension instead of building the result manually.
- Replace `item["active"] == True` with `item.get("active") is True` if the field is expected to be the Boolean `True`.
- Add backoff between attempts. A fixed two-second delay can make retries slower than necessary.
- Decide whether failures should raise an exception or return `[]`. Returning `[]` makes a failed request indistinguishable from a suc…
16.5s1713 output tokens
Frequently asked questions
- Which is cheaper, GPT-5.6 Sol or GPT-6 Luna?
- Per 1M output tokens, GPT-5.6 Sol is ¥1,787 and GPT-6 Luna is ¥89.35 on FastMetal (yen, before tax), so GPT-6 Luna is cheaper.
- How do the context windows of GPT-5.6 Sol and GPT-6 Luna compare?
- GPT-5.6 Sol takes 1,050,000 tokens; GPT-6 Luna takes 1,050,000.
- Can I use GPT-5.6 Sol and GPT-6 Luna with the same API key?
- Yes. On FastMetal's OpenAI-compatible endpoint, switch by passing "gpt-5.6-sol" or "gpt-6-luna" as the model. Each is billed at its own rate from the same prepaid balance.
Try both on one API key
Create an account and add credit to call GPT-5.6 Sol and GPT-6 Luna from the browser chat and the API. No monthly fee.